The present invention relates to methods for recovering rare earth elements, and in particular, membrane assisted solvent extraction for the recovery of rare earth elements from post-consumer products and other end-of-life products.
Rare earth elements play an increasing role in the development of green energy and in high-tech industries. For example, the demand for rare earth elements has grown in response to an increased use of permanent magnets for electric motors, rechargeable batteries for hybrid electric vehicles, catalysts for petroleum refining, phosphors in flat panel displays, and generators for wind turbines.
Rare earth elements include a group of fifteen lanthanide elements along with scandium and yttrium. Currently, post-consumer products that include rare earth elements include the following: 1) permanent NdFeB magnets (neodymium (Nd), dysprosium (Dy), praseodymium (Pr)) in automobiles, mobile phones, hard disk drives, computers, consumer electronic devices, industrial electric motors, hybrid electric vehicles; 2) phosphors (europium (Eu), terbium (Tb), yttrium (Y)) in fluorescent lamps, LEDs, LCD backlights, plasma screens, cathode-ray tubes; and 3) nickel metal hydride batteries (lanthanum (La), cerium (Ce), Nd, Pr) in rechargeable batteries and in hybrid electric vehicle batteries. However, less than 1% of these rare earth elements are being recycled due to low efficiencies in existing recovery processes.
Currently recovery processes for rare earth elements include hydrometallurgy, pyrometallurgy, gas-phase extraction, and solvent extraction. Among these processes, hydrometallurgy is the most commonly used recovery process for permanent magnets. For example, permanent magnets can be dissolved in strong acids such as sulfuric acid, hydrochloric acid, phosphoric acid, and nitric acid, and the rare earth elements can be selectively precipitated as double sulfates, oxalates, and fluorides. The major challenges of hydrometallurgical processes, however, are high chemical usage, low selectivity due to co-extraction of non-rare earth elements, and generation of large amounts of waste. The rare earth elements can also be recovered by pyrometallurgical processes involving re-melting or liquid metal extraction from transition metals in the metallic state. However, this process creates slag formation and loss of a large amount of rare earth elements due to the carbon and oxygen contents in the scraps. In addition, pyrometallurgical processes require further separation for the recovered mixture of rare earth elements and high investment cost for high temperature furnaces.
Gas-phase extraction, as noted above, has also been proposed for the recovery of rare earth elements. Gas-phase extraction involves the separation of rare earth elements based on volatility differences, involving chlorination and carbochlorination with Cl2 and CO in a N2 stream. However, this process generates highly corrosive aluminum chloride with the formation of hydrogen chloride gas. Solvent extraction is another approach to recover rare earth elements by using the different solubilities of solutes in two immiscible liquids. For example, Pr and Nd were efficiently extracted via this process with 10% saponified Cyanex 272 (Bis(2,4,4-trimethylpentyl) phosphinic acid) and 0.5M TBP (tri butyl phosphate). In the conventional solvent extraction processes, however, separation is limited by the equilibrium of substances, requiring contact time enough for the dispersion of one phase in another immiscible phase. Additionally, this equilibrium-based separation process carries out extraction and stripping in two separate steps, and requires loading, flooding, third phase formation, and extractant loss as part of the overall recovery process.
Accordingly, there remains a need for an improved system and method for the recovery of rare earth elements. In particular, there remains a need for an improved system and method for the selective recovery of rare earth elements from post-consumer products and other end-of-life products while also reducing the environmental impact associated with existing rare earth element recovery processes. It is also desirable to recover rare earth elements in a highly pure form that is suitable for direct reuse or recycling with minimal or no post-processing.